feat(api): Add SDN zone listing endpoint

## Context

The proxmox_controller supports `network_list_node_sdn_zones` but it is not exposed via API. Useful for network topology planning in the UI.

## Requirements

- `GET /v0/admin/proxmox/network/node/{node}/sdn-zones` → `network_list_node_sdn_zones`

## Priority

Nice-to-have.

## References

- Gap analysis: `gap-analysis.md` issue B19